The Fix
Upgrade to version 0.18.0 or later.
Based on closed Kludex/uvicorn issue #1305 · PR/commit linked
Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.
@@ -1,5 +1,8 @@
-e .[standard]
+# Type annotation
+asgiref==3.5.2
+
repro.py
try:
from asgiref.typing import WebSocketScope
except ImportError:
from asgiref.typing import WebsocketScope
WebSocketScope = WebsocketScope
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
fix.md
Option A — Upgrade to fixed release\nUpgrade to version 0.18.0 or later.\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\nOption C — Workaround\nuntil https://github.com/django/channels/issues/1722 is resolved?\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n
Why This Fix Works in Production
- Trigger: Error: 1-03 17:27:52 +0000] [9] [ERROR] Exception in ASGI application
- Mechanism: Removes the `asgiref` dependency and adds it to the `requirements.txt`, addressing the backward compatibility issue.
- Why the fix works: Removes the `asgiref` dependency and adds it to the `requirements.txt`, addressing the backward compatibility issue. (first fixed release: 0.18.0).
Production impact:
- If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.
Why This Breaks in Prod
- Shows up under Python 3.10 in real deployments (not just unit tests).
- Surfaces as: Error: 1-03 17:27:52 +0000] [9] [ERROR] Exception in ASGI application
Proof / Evidence
- GitHub issue: #1305
- Fix PR: https://github.com/kludex/uvicorn/pull/1532
- First fixed release: 0.18.0
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.85
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.51
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I'm not sure @Tony - that's a good question”
“I will keep this issue open for now > I'd certainly suggest closing #1373 off, yes. Can you clarify what's meant by closing it off?…”
“> Can you clarify what's meant by closing it off? Yes”
“I'm willing to approve a PR with the vendor, and tests running against asgiref.typing. :+1:”
Failure Signature (Search String)
- Error: 1-03 17:27:52 +0000] [9] [ERROR] Exception in ASGI application
Error Message
Stack trace
error.txt
Error Message
-------------
Error: 1-03 17:27:52 +0000] [9] [ERROR] Exception in ASGI application
Traceback (most recent call last):
File "/opt/venv/lib/python3.10/site-packages/uvicorn/protocols/http/httptools_impl.py", line 376, in run_asgi
result = await app(self.scope, self.receive, self.send)
File "/opt/venv/lib/python3.10/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
return await self.app(scope, receive, send)
File "/opt/venv/lib/python3.10/site-packages/uvicorn/middleware/asgi2.py", line 17, in __call__
await instance(receive, send)
File "/opt/venv/lib/python3.10/site-packages/channels/http.py", line 192, in __call__
await self.handle(body_stream)
File "/opt/venv/lib/python3.10/site-packages/asgiref/sync.py", line 409, in __call__
raise RuntimeError(
RuntimeError: Single thread executor already being used, would deadlock
Minimal Reproduction
repro.py
try:
from asgiref.typing import WebSocketScope
except ImportError:
from asgiref.typing import WebsocketScope
WebSocketScope = WebsocketScope
Environment
- Python: 3.10
What Broke
Users experienced deadlock errors when serving static files under heavy load.
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
Upgrade to version 0.18.0 or later.
When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.
Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.
Option C — Workaround Temporary workaround
until https://github.com/django/channels/issues/1722 is resolved?
When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.
Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.
Option D — Guard side-effects with OnceOnly Guardrail for side-effects
Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.
- Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
- Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
- Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
Show example snippet (optional)
onceonly.py
from onceonly import OnceOnly
import os
once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)
# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..." # replace
key = f"stripe:webhook:{event_id}"
res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
return {"status": "already_processed"}
# Safe to execute the side-effect exactly once.
handle_event(event_id)
When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Fix reference: https://github.com/kludex/uvicorn/pull/1532
First fixed release: 0.18.0
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use if it changes public behavior or if the failure cannot be reproduced.
- Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.
Verify Fix
verify
Re-run the minimal reproduction on your broken version, then apply the fix and re-run.
Did This Fix Work in Your Case?
Quick signal helps us prioritize which fixes to verify and improve.
Prevention
- Add a stress test that runs high-concurrency workloads and fails on thread dumps / blocked locks.
- Enable watchdog dumps in prod (faulthandler, thread dump endpoint) to capture deadlocks quickly.
Version Compatibility Table
| Version | Status |
|---|---|
| 0.18.0 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.